Critical data is often copied to another storage site to protect against disasters that may result in data loss. One such technique for this purpose is known as remote copying where a disk (or logical volume) is paired with another disk for use as a backup. The original disk is known as the primary and the backup disk is known as the secondary. Whenever data is written to the primary it must also be written to the secondary to ensure the backup stays up to date. Remote copying may be implemented synchronously so that processing at the host is delayed until confirmation of the completion of the corresponding write at the secondary. Remote copying may be also implemented asynchronously such that the host that wrote the data to the primary storage is not delayed while data is copied to the secondary storage. That is, as soon as the data has been written to the primary storage, the host is notified of its completion. The data is then copied to the secondary storage asynchronously. In some instances, replicated writes in a set of related disks are assigned a sequence number according to the order the host submits them. The writes are replicated to a remote site or backup site and applied by sequence number order to ensure that the disks are always mutually consistent.